1. Field of the Invention
The present invention relates to an inclination detecting apparatus and an inclination detecting method for detecting an inclination of a surface of an object to be detected relative to a certain reference plane. More particularly, the present invention relates to an inclination detecting apparatus and an inclination detecting method suitable for applications in exposure apparatus used for production of semiconductor devices and liquid crystal display devices.
2. Related Background Art
Generally, semiconductor devices, liquid crystal display devices or the like are produced by the photolithography using a reduction projection type exposure apparatus, especially using a step-and-repeat exposure apparatus or stepper, which projects an image of pattern on photomask or reticle (as will be referred to generally as "reticle") through a projection optical system onto a photosensitive substrate to effect exposure thereon. In such exposure apparatus, since the exposure pattern is very fine, the numerical aperture (NA) of the projection optical system is large and the permissible focal depth is very shallow. Therefore, in order to effect high-resolution exposure of reticle pattern image on an exposed surface of the photosensitive substrate, the exposed surface must be maintained in parallel with the best focus plane of the projection optical system within the permissible focal depth. Thus, in the case of using, for example, a wafer coated with photoresist thereon as the photosensitive substrate, conventionally, the height (position along the optical axis of projection optical system) is measured at least at three points on the wafer and the entire exposed surface of wafer is positioned approximately perpendicular to the optical axis of projection optical system.
However, if the wafer is large in scale or if the flatness of wafer itself is poor, the height detection must be conducted in a partial area on the exposed surface of wafer. Since deformation of wafer is further enhanced by exposure and chemical treatments for layers on the wafer, the inclination detection becomes indispensable in order to accurately detect an inclination of a surface in the exposed area on the wafer with respect to the image plane of projection optical system.
There are conventional inclination detecting apparatus for example as disclosed in U.S. Pat. Nos. 4,558,949 and 4,902,900, which can maintain exposed areas on the wafer respectively in the best condition by detecting an average surface inclination for each area. The apparatus as disclosed have very excellent measurement performance, by which the normality can be accurately measured relative to the optical axis of projection optical system for each exposed area on wafer in projection exposure apparatus.
There are, however, problems recognized as follows in the conventional inclination detecting apparatus as described above. If an object to be inclination-detected is a resist-coated transparent glass wafer, especially if an object is a resist-coated transparent glass wafer having a film with high reflectivity, such as aluminum, formed on the back surface thereof, light reflected from the back surface of glass wafer is mixed into light reflected from the front surface of glass wafer, which could cause a problem of error in inclination detection. Further, the reflected light from the back surface of glass wafer may be reflected by the front surface of glass wafer to return to the back surface and then to be again reflected by the back surface, which could be repeated several times between the back surface and the front surface of glass wafer, finally passing through the front surface of glass wafer to impinge on a photodetector. This could increase the error in inclination detection.